Generally, rectifiers are used for the conversion of AC to DC voltage. A CMOS bridge circuit 100 that can be used in such a rectifier is shown in FIG. 1. The CMOS bridge circuit 100 can be regarded as a non-linear, two-port device having an input voltage u1(t) that receives the AC voltage, a output voltage U2, and four CMOS switches PMOS1, PMOS2, NMOS1, and NMOS2. In general, the output port is connected to a load which may be a purely resistive load (full-wave rectifier), or a resistive load in parallel with a capacitive load (for DC voltage).
The gates of the CMOS switches may be directly connected to the input voltage terminals. Assuming a purely resistive load and an ideal switching performance of the transistors, the following conditions are fulfilled:U2=|u1 (t)|, if |u1(t)|≧uTHR, andU2=0, if |u1(t)<uTHR,whereby voltage uTHR denotes a MOS-threshold voltage, which here is assumed to be equal for both, PMOS and NMOS transistors. For u1(t)≧uTHR, PMOS1 and NMOS2 are switched on (low impedance), whereas transistor PMOS2 and NMOS1 are switched off (high impedance), and vice versa for u1(t)≦−uTHR, transistors PMOS2 and NMOS1 are switched on, and transistors PMOS1 and NMOS2 are switched off. Thus, for the special case of an ohmic load, the CMOS-bridge 100 of FIG. 1 represents a full-wave rectifier. Note that here the full input voltage magnitude applies at the load and there is no reduction due to diode voltage drops. Typically, MOS threshold voltages are uTHR˜0.7V.
Assuming a sinusoidal input voltage, the bridge circuit 100 does not fully work as a rectifier for all types of loads because transistor switches operated in ON-states allow current flow in both directions (in contrast to a diode). For example, if the circuit load is a parallel resistor and capacitor then the capacitor is partly discharged by the transistors in switch-turn-on states. Assuming u1(t)>uTHR, PMOS1 and NMOS2 are switched on, and voltage U2 simply follows the input voltage u1(t). This means that the load capacitor is discharged not only via the load resistor, but also via the input lines. One way to address this is connect an output diode in series with the load resistor and output capacitor; however this may be unacceptable in low power applications due to the diode voltage drop.
Furthermore, when a constant DC voltage is desired, the addition of a smoothing capacitor and/or diode in conjunction with the bridge circuit may be impractical for applications that have limited space. For example, the sensing/stimulation element of a retinal implant is positioned directly within the eyeball, making space a primary concern.